This invention relates to a squelch signal generator for use in a radio receiver responsive to a radio signal carrying an input signal of a baseband. It is noted here throughout the instant specification that the input signal is a digital signal, such as a binary signal, a ternary signal, or the like.
In general, a squelch signal generator of the type described is put into operation to classify such an input signal into a desired signal and an undesired signal and to produce a squelch signal when the input signal is classified into the undesired signal. The desired signal may be derived from a specific radio channel assigned to a radio receiver including the squelch signal generator in question while the undesired signal falls within the other radio channels except the specific radio channel.
A conventional squelch signal generator detects a field intensity of the specific radio channel and produces a squelch signal when the field intensity is lower than a preselected intensity which may be called a squelch level. Another conventional squelch signal generator detects a noise level interspersed in the other channels and produces a squelch signal when the noise level is higher than a preselected noise level which may also be called a squelch level.
However, both of the conventional squelch signal generators are disadvantageous in that a malfunction inevitably takes place due to interference and an intermodulation noise which result from the other channels. In other words, the squelch signal is wrongly interrupted with each of the conventional squelch signal generators despite the fact that the desired signal is not received. This results in occurrence of noise on reproduction of the input signal.
In addition, each squelch level fluctuates due to a variation of temperature and aging in the conventional squelch signal generators. The fluctuation of the squelch signal level brings about a variation of a bit error rate or symbol error rate to be detected by the squelch signal generators. This means that the squelch signal is not produced from either of the conventional squelch signal generators when the input signal has a high bit error or a high symbol error rate. The bit error rate and the symbol error rate will collectively be named an error rate.